A conserved spider silk domain acts as a molecular switch that controls fibre assembly

Spider silk's dual identity Many proteins form fibrillar structures at high concentrations, but spider silk proteins, with highly repetitive segments flanked by non-repetitive (NR) terminal domains, behave differently. They are remarkably soluble when stored at high concentration yet can convert to extremely sturdy fibres on demand. The molecular mechanism that makes this possible is not yet clear, but two structural studies in this issue provide new clues. Askarieh et al. present the 1.7 Å X-ray crystal structure of the N-terminal domain of a dragline spidroin from the nursery web spider Euprosthenops australis. The structure shows how this highly conserved domain can regulate silk assembly by preventing premature aggregation of spidroins and triggering polymerization as the pH falls along the silk extrusion duct. Hagn et al. determined the solution structure of the C-terminal NR domain of the dragline silk protein fibroin 3 from the common orb-weaver Araneus diadematus. They observe a conformational switch, activated by chemical or mechanical stimuli, between storage and assembly forms of the protein.